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I'm not sure where you're getting this...
I just ran in this command:
dd if=/dev/urandom of=test bs=1M count=10000
on Ubuntu 10.04 on a Core i3 530, which is relatively slow. I noticed no decrease in responsiveness whatsoever. I even tried playing an OpenGL game. The FPS dropped by a few frames, but it was not choppy or unresponsive at all.
Maybe it's just your particular system that has problems?
(Note: I'm not trying to say that there is a problem with X11. I'm saying that the problem doesn't exist, in Linux or X11.)
Every system we try this on at work (Kubuntu 10.04, Kubuntu 9.10, Debian 5.0, Debian 4.0) will grind to a halt until the dd command is done. You can't CTRL+TAB to other windows (well, you can, but it takes 30-45 seconds to register the keypress), you can't load other apps, you basically just stare at the blinky HD light.
These are Intel P4, AMD Sempron, desktops and Intel Core2 Duo laptops, using ATi Radeon X-series PCie, nVidia something PCIe, nVidia onboard, and Intel onboard graphics.
Every system we try it on is affected, even our multi-core Opterons with 3Ware RAID controllers and no GUI (keyboard lag, 30s or more to ALT+F* switch consoles, etc).
Try it with a USB stick as the of= option to really cause havoc.
Our sys analyst is a RedHat guy, and he's avoided dd for years because of this issue (RedHat 7-9, Fedora something, RHEL 4/5).
The last time I tried this was to make a bootable ChromiumOS USB stick. Took 20 minutes to dd the stick ... during which time I could not use my desktop.
This Linux bug was my biggest problem on my laptop running Linux in 1 GB RAM.
It has absolutely nothing to do with the scheduler and everything to do with the virtual memory and disk systems.
The VM and disk get so busy writing data out to disk that the VM has no free pages and the disk is too busy to read in data blocks.
Heaven help you if the system starts to swap.
People are working to improve this in every Linux release but it's been a big problem since 2.6 was released.